The proposed work is aimed at developing a selective MMP-14 probe for detecting metastatic and pro-tumorigenic cancer cells characterized by the over-expression of MMP-14. This will be achieved by optimizing a chimeric heterobivalent platform targeted simultaneously to two separate domains of MMP-14. Ligands to both the active site of the catalytic domain and the hemopexin domain will be tethered through a modular linker unit that can be varied in length to match the distance between the two domains. In a solvent exposed region of the linker, a near-IR dye will be installed to allow the heterobivalent platform to selectively label MMP-14- positive cells for detection by confocal microscopy and in vivo optical imaging. This achievement is important because it will ensure the feasibility of advancing this technology to PET imaging applications. In all likelihood, this general strategy can be applied to the selective targeting of other MMPs that serve as disease biomarkers. Our central hypothesis is that a high-affinity MMP-14-specific heterobivalent platform can be developed by optimizing the linker length between a MMP catalytic domain inhibitor and hemopexin domain ligand. When outfitted with a fluorescent dye, this chimeric platform will selectively label MMP-14-positive cells for in vitro and in vivo detection by confocal microscopy and optical imaging, respectively. The rationale for undertaking the proposed research is that the MMP-14-specific imaging platform will serve as the foundation for developing a clinically-relevant imaging modality for the diagnosis and post-treatment assessment of breast and other tumors characterized by over-expression of MMP-14. The approach is innovative in that it aims to selectively target the membrane-bound MMP-14 with a heterobivalent platform tailored to the distance between two of MMP-14's targetable domains rather than optimizing the affinity of scaffolds targeted to a single binding site. In Aim #1, we will test the working hypothesis that optimizing the linker length between a MMP catalytic domain inhibitor and hemopexin domain ligand will result in greater and selective affinity for MMP-14 than monovalent ligands or inhibitors. It is our expectation that the distance between the two domains will provide a unique and additional specificity element to the proposed chimeric heterobivalent platform, which can be tailored to individual MMPs, including MMP-14. Aim 2 is focused on proving-the-concept that enhanced binding to MMP-14 can be accomplished with heterobivalent platform and that fluorescently-labeling this platform with a near-IR fluorescent dye will enable optical in vivo imaging of MMP-14-positive tumors. At the conclusion of these studies, it is our expectation that a working probe for targeting MMP-14-positive tumors will be available for subsequent radiolabeling and the development of a clinically-relevant PET imaging agent for pro-metastatic tumors.